The technique of combined Gas Chromatography/Mass Spectrometry (GC/MS) is considered to be the most powerful and definitive technique for the analysis of trace organics in the environment, as well as other fields such as pharmaceuticals and forensic science. Combined Liquid Chromatography/Mass Spectrometry (LC/MS) is gaining significance as a complimentary tool to GC/MS since liquid chromatography (LC) can often separate trace organics that are incompatible with gas chromatography (GC). Organic compounds for which LC provides a distinct advantage are characterized by either high molecular weight, low volatility, high polarity, sensitivity toward thermal degradation or some combination of the above. Many pharmaceuticals and toxic organic compounds fall into this classification. Conventional mass spectrometry requires that organic compounds exist in the gas phase in order to allow ionization, mass separation and detection. This requirement provides an ideal marriage between GC and MS but not between LC and MS. Although the potential for LC/MS is greater than that presently enjoyed by GC/MS, growth in LC/MS has been limited by the inherent phase incompatibility of liquid chromatography and the gas phase and vacuum requirements of mass spectrometry.
The underlying requirement of most LC/MS techniques is to transform the liquid HPLC eluent, consisting of solvent and chromatographically separated solute, into the gas phase. This transformation is necessary in order to separate the solvent from the analytically important solute, or to use the solvent vapor as a chemical ionization reagent. Additionally, most mass spectrometers and LC/MS interfaces require that the solute and solvent be present in the gas phase in order to ionize the solute and, in most cases, accommodate the vacuum requirements of the mass spectrometer. Although novel ionization techniques and ionization at atmospheric pressure have been described in conjunction with LC/MS, volatilization of the HPLC solvent remains a crucial and common feature of all LC/MS interfaces.
Reverse phase LC (mobile phase more polar than the stationary phase) is the LC method of choice for many organic compounds that are incompatible with GC. The mobile phase (eluent) is often water or a gradient mixture of water:methanol or water:acetonitrile. Water as a mobile phase or mobile phase component presents a particularly difficult problem for an LC/MS interface due to the unusually high latent heat of vaporization of water.
Most current LC/MS interface techniques operate in two steps. The first step transforms the liquid eluent into an aerosol which increases the available surface area for subsequent vaporization. The second step involves desolvation (evaporation) of the aerosol by application of heat. Many LC/MS interface techniques rely on a narrow and consistent aerosol size distribution in order to ionize the solute or electrostatically focus preformed ions immediately following the moment of complete vaporization. The close proximity in time between vaporization and ionization is often necessary in order to avoid overheating of the solute in the desolvation step. Control of the aerosol and extent of evaporation becomes difficult with gradient elution techniques because of the continuous change in solvent composition. Binary or ternary gradient elution reverse phase liquid chromatography is widely used with conventional detectors such as ultraviolet absorbance and fluorescence in order to optimize the chromatography for a particular application.
A variety of LC/MS interfaces are well-known in the patented prior art. McLafferty et al (U. S. Pat. No. 3,997,298 Dec. 14, 1976) describe a direct liquid introduction (DLI) which utilizes a restricted capillary to introduce liquid HPLC eluent directly into a heated chemical ionization source of a mass spectrometer. The solvent is vaporized in the ion source and serves as a chemical ionization reagent. This approach is limited to HPLC eluent flow rates on the order of 0.010-0.012 ml/min. Since conventional HPLC flow rates are on the order of 0.5-1.5 ml/min, the large majority of the HPLC eluent must be diverted away from the mass spectrometer.
McFadden (U.S. Pat. No. 4,055,987 Nov. 1, 1977) describes an LC/MS interface in which the liquid eluent is deposited onto a continuous moving ribbon. The ribbon passes through vacuum locks which, with the assistance of radiant heat, strip away the volatile solvent. The solutes are then thermally desorbed from the ribbon into the ion source of the mass spectrometer. A similar device has been described by Andresen et al (U.S. Pat. No. 4,740,298 Apr. 26, 1988). Although this approach can accommodate a larger fraction of conventional HPLC eluent flows than direct liquid introduction, the use of an aqueous mobile phase requires elevated temperatures which may decompose thermally sensitive compounds in the solvent stripping or thermal desorption steps.
Miyagi et al (U.S. Pat. No. 4,112,297 Sept. 5, 1978) describe the use of an ultrasonic nebulizer to convert the HPLC eluent into an aerosol. This interface provides a nebulization efficiency of 5-10 percent at an eluent flow of 1.0 ml/min. The nebulized fraction of the eluent is mixed with a carrier gas and passed through a heated cavity to evaporate the solvent component of the aerosol. The vapor phase is removed and the remaining particles are introduced into the mass spectrometer ion source.
Fite (U.S. Pat. No. 4,209,696 June 24, 1980) describe an electrospray LC/MS interface in which the HPLC eluent is passed through an electrically conductive capillary maintained at a high voltage relative to the surroundings at the exit of the capillary. This voltage causes the eluent to emerge from the capillary as fine droplets which are electrically charged. The charged droplets are passed through a heated cavity to evaporate the droplets resulting in neutral solvent vapor and ionized solute molecules.
White et al (U.S. Pat. No. 4,281,246 July 28, 1981) describe the use of a stationary tapered wire concentrator in which the eluent flows down the wire while a portion of the solvent is evaporated. The concentrated eluent is introduced into the mass spectrometer in a direct liquid introduction manner.
Takeuchi et al (U.S. Pat. No. 4,298,795 Nov. 3, 1985) describe an LC/MS interface which converts the HPLC eluent into an aerosol by nebulization with helium gas. The aerosol is optionally passed through a cavity heated at 300.degree. C. to evaporate the solvent portion of the aerosol. The concentrated aerosol and solvent vapor are either introduced directly into the mass spectrometer ion source or passed through a jet type separator prior to the ion source in order to remove the majority of the solvent vapor.
Labowsky et al (U.S. Pat. No. 4,531,056 July 23. 1985) describe an electrospray LC/MS interface, similar to that of Fite, in which the fine droplets are desolvated by heated inert gas and ionized at atmospheric pressure.
Sakairi et al (U.S. Pat. No. 4,750,068 Feb. 11, 1986) describe an ultrasonic nebulization LC/MS interface for use with an atmospheric pressure ionization source. The nebulized aerosol is passed through a heated cavity to evaporate the solvent.
Vestal et al (U.S. Pat. No. 4,730,111 Mar. 8, 1988) describe four embodiments of a "thermospray" LC/MS interface in which solvent evaporation occurs in two steps. The first step involves partial evaporation of the HPLC eluent by passage through a heated stainless steel capillary. The second step involves passage of the aerosol through a heated cavity to complete the vaporization process. The addition or incorporation of a volatile salt such as ammonium acetate in the eluent results in an unbalanced charge on the particles as the solvent evaporates. This unbalanced charge in part results in ionization of the solute molecules. Heating of the capillary is accomplished by embedding a portion of the capillary in a heated copper block (embodiments 1 and 2), a combination of a heated copper block and electrical resistance heating of the stainless steel capillary (embodiment 3) and electrical resistance heating of the entire stainless steel capillary (embodiment 4).
Embodiment 4 is the preferred embodiment due to the low thermal mass of the stainless steel capillary. The low thermal mass of the capillary allows the interface to accommodate changing eluent composition or flow rates by alteration of the electrical power to the capillary. Although complete vaporization within the capillary can be achieved, this mode is avoided since the exit of the capillary can easily and greatly exceed the normal operating temperature of approximately 200.degree. C. at an aqueous eluent flow of 1.0 ml/min. The nonuniformity of the capillary temperature is due to the need to provide sufficient electrical power to accommodate the high latent heat of vaporization of the aqueous mobile phase. Once the aqueous mobile phase is partially or completely vaporized, the exit region of the capillary cannot dissipate the power resulting is a dramatic rise in capillary temperature at the exit region. The resulting excessive temperature of the exit gas may result in degradation or pyrolysis of thermally sensitive solutes. A variety of temperature and flow feedback controls together with recognition of the gradient composition and rate are of critical necessity in order to control the electrical power to accommodate variations in HPLC eluent flow and eluent composition.
Several LC/MS interfaces have been described in the literature, however, most are adaptations of the above prior art and are not relevant to this invention. Two noteworthy review articles have appeared (T. R. Covey, E. D. Lee, A. P. Bruins, J. D. Henion, Anal. Chem. 1986, 58, pp. 1451A-1461A and J. B. Crowther, T. R. Covey, D. Silvestre, J. D. Henion, LC. 1985, 3, pp. 240-254) which trace the historical development of LC/MS interfaces and provide a summary of existing state-of-the art.
A further significant development is a monodisperse aerosol generation interface for LC/MS (MAGIC) (R. C. Willoughby, R. F. Browner, Anal. Chem. 1984, 56, pp. 2626-2631, and P. C. Winkler, D. D. Perkins. W. K. Williams, R. F. Browner, Anal. Chem. 1988, 60, pp. 489-493). This work describes a method for isolating solute aerosol particles from a helium nebulized LC eluent. The nebulized eluent passes through a heated cavity to evaporate the volatile solvent which results in lesser volatile solute aerosol particles. The aerosol particles are collimated at the exit of the heated cavity and emitted through a 0.5 mm aperture with the solvent vapor and helium nebulizing gas into a series of vacuum regions containing skimmers. The solute particles maintain a collimated beam along the axis of the skimmers due to their relatively high radial inertia. The solvent molecules and helium gas do not maintain the high degree of beam collimation, are excluded by the skimmers, and are pumped away by the vacuum system. The particles then impact the wall of the heated ion source resulting in flash vaporization and subsequent ionization. The use of skimmers in mass spectrometers is well-known in molecular beam applications and has been applied elsewhere to mass spectrometry analysis of aerosol particles (J. Allen, R. K. Gould, Rev. Sci. Instrum. 1981, 52, pp. 804-809).
Browner et al (R. F. Browner, A. W. Born, Anal. Chem. 1984, 56, pp. 875A-888A) describe a variety of aqueous sample introduction techniques for atomic spectroscopy which involve gas or ultrasonic nebulization. None of the approaches described are relevant to the present invention.